Interface circuit, transmission system, and transmission direction control method

ABSTRACT

Bidirectional transmission is performed by switching transmission directions of a serial transmission path. A transmission system includes the first device that includes a transmission circuit which performs transmission to the second device through the transmission path, and a transmission direction reversal control unit which opens the transmission path after making a request for reversal of a transmission direction in the transmission path to the second device, and reverses the transmission direction at the transmission circuit in response to permission for reversal from the second device. The second device includes a transmission circuit which performs transmission to the first device through the transmission path, and a transmission direction reversal control unit which opens the transmission path in response to the request for reversal from the first device, and, when the first device becomes accessible, gives the first device the permission for reversing the transmission direction at the second transmission circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase of International PatentApplication No. PCT/JP2015/059040 filed on Mar. 25, 2015, which claimspriority benefit of Japanese Patent Application No. JP 2014-137964 filedin the Japan Patent Office on Jul. 3, 2014. Each of the above-referencedapplications is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present technology relates to a transmission system which performstransmission between devices through a transmission path. Specifically,the present technology relates to an interface circuit which controls atransmission direction in a transmission path, a transmission system,processing methods therein, and programs which cause a computer toexecute the methods.

BACKGROUND ART

In recent years, with the spread of audio/visual (AV) devices whichprocess digital signals, such as sound signals, video signals, and thelike, interfaces employing a variety of methods have been suggested asinterfaces for transmitting digital signals between such AV devices. Assuch an interface, for example, high-definition multimedia interface(HDMI) standard (HDMI is a registered trademark) is widely known. In aninterface such as HDMI, a unidirectional serial transmission path isused to transmit a large amount of data at high speed (for example, seePatent Literature 1).

CITATION LIST Patent Literature

Patent Literature 1: JP 2009-130606A

SUMMARY OF INVENTION Technical Problem

In the related art described above, a unidirectional serial transmissionpath is used. Therefore, in order to perform high speed transmission intwo directions, it is necessary to provide a plurality of interfaces.These days, provision of content through a network is increasing, and,for example, when a television receiver acquires content from a networkand plays the content, there is a use case of desiring to returnhigh-quality audio to an amplifier side. In HDMI, a function oftransmitting audio in a reverse direction through an audio returnchannel (ARC) has been defined already, but neither transmission of datarequiring a broadband nor transmission of multichannel LPCM aresupported.

The present technology has been created in consideration of such asituation, and intends to perform bidirectional transmission byswitching transmission directions of a serial transmission path.

Solution to Problem

The present technology has been made in view of solving the aboveproblem, and a first aspect thereof provides an interface circuitincluding: a transmission circuit configured to perform transmission toanother device through a transmission path; and a transmission directionreversal control unit configured to open the transmission path aftermaking a request for reversal of a transmission direction in thetransmission path to the other device, and to reverse the transmissiondirection at the transmission circuit in response to permission forreversal from the other device, a transmission direction control methodfor the interface circuit, and a program. With this, by making a requestfor reversal of a transmission direction in a transmission path, theeffect of reversing the transmission direction is achieved.

Further, in the first aspect, while the transmission circuit transmitsdata to the other device, the transmission direction reversal controlunit may make the request for reversal to the other device. Further,when the request for reversal is received from the other device whilethe transmission circuit transmits data to the other device, thetransmission direction reversal control unit may make the request forreversal to the other device. Moreover, the transmission circuit mayinclude an amplifier alternating-current connected to a differentialinput from the transmission path.

Further, a second aspect of the present technology provides an interfacecircuit including: a transmission circuit configured to performtransmission to another device through a transmission path; and atransmission direction reversal control unit configured to open thetransmission path in response to a request for reversal of atransmission direction in the transmission path from the other deviceand, when the other device becomes accessible again, give the otherdevice permission for reversal and reverse the transmission direction atthe transmission circuit, a transmission direction control method forthe interface circuit, and a program. With this, the effect of reversingthe transmission direction is achieved in response to a request forreversal of a transmission direction in a transmission path. Here, thetransmission circuit may include an amplifier which isalternating-current connected to a differential output to thetransmission path.

Further, in the second aspect, while the transmission circuit receivesdata from the other device, the transmission direction reversal controlunit may receive the request for reversal from the other device.Further, while the transmission circuit receives data from the otherdevice, the transmission direction reversal control unit may make therequest for reversal to the other device.

Further, a third aspect of the present technology provides atransmission system including: a first device; and a second device, thefirst device and the second device performing transmission to each otherthrough a transmission path. The first device includes a firsttransmission circuit configured to perform transmission to the seconddevice through the transmission path, and a first transmission directionreversal control unit configured to open the transmission path aftermaking a request for reversal of a transmission direction in thetransmission path to the second device, and to reverse the transmissiondirection at the first transmission circuit in response to permissionfor reversal from the second device, and the second device includes asecond transmission circuit configured to perform transmission to thefirst device through the transmission path, and a second transmissiondirection reversal control unit configured to open the transmission pathin response to the request for reversal from the first device, and, whenthe first device becomes accessible again, to give the first device thepermission for reversal and to reverse the transmission direction at thesecond transmission circuit. With this, a first device makes a requestfor reversal of a transmission direction in a transmission path, and asecond device gives permission for reversal so that the effect ofreversing the transmission direction is achieved.

Advantageous Effects of Invention

According to the present technology, it is possible to achieve anexcellent effect of being able to perform bidirectional transmission byswitching transmission directions of a serial transmission path. Also,effects stated herein are not necessarily limited, and any effectsstated in the present disclosure may exist.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual configuration diagram of a transmission system inaccordance with an embodiment of the present technology.

FIG. 2 is a diagram showing an example of a pin arrangement of aconnector in accordance with an embodiment of the present technology.

FIG. 3 is a diagram showing an example of a functional configurationrequired to control reversal of a transmission direction in accordancewith an embodiment of the present invention.

FIG. 4 is a diagram showing examples of configurations of TMDStransmission circuits 101 and 201 in accordance with an embodiment ofthe present invention.

FIG. 5 is a diagram showing an example of a configuration of device A100 in accordance with an embodiment of the present technology.

FIG. 6 is a diagram showing an example of a configuration of device B200 in accordance with an embodiment of the present technology.

FIGS. 7a, 7b and 7c are a diagram showing an example of a format of aTMDS transmission direction reversal requesting register in accordancewith an embodiment of the present invention.

FIG. 8 is a flowchart showing an example of a processing procedure whena source device dominantly controls reversal of a TMDS transmissiondirection in accordance with an embodiment of the present invention.

FIG. 9 is a flowchart showing an example of a processing procedure whena sync device dominantly controls reversal of a TMDS transmissiondirection in accordance with an embodiment of the present invention.

FIGS. 10a, 10b and 10c are a diagram showing an example of aconfiguration of an audiovisual system in accordance with an embodimentof the present technology.

FIG. 11 is a diagram showing an example of a configuration of a discplayer 500 in accordance with an embodiment of the present technology.

FIG. 12 is a diagram showing an example of a configuration of atelevision receiver 600 in accordance with an embodiment of the presenttechnology.

FIG. 13 is a diagram showing an example of a configuration of a digitalcamera 700 in accordance with an embodiment of the present technology.

DESCRIPTION OF EMBODIMENT(S)

Hereinafter, a mode for implementing the present technology (referred toas an embodiment below) will be described. Description will be made inthe following order.

1. Embodiment (example of transmission direction reversal control)

2. Application example

1. Embodiment

[Interface]

FIG. 1 is a conceptual configuration diagram of a transmission system inaccordance with an embodiment of the present technology. Thistransmission system adopts an HDMI standard as a basic configuration andhas a function of switching transmission directions of transitionminimized differential signaling (TMDS) transmission. In the HDMIstandard, one direction is determined as a transmission directionaccording to a basic high-speed transmission line, and a device on atransmitting side and a device on a receiving side are referred to as asource device and a sync device, respectively. In this example, device A100 and device B 200 are connected by means of a cable 300. When deviceA 100 is the source device, device B 200 is the sync device, and whendevice B 200 is the source device, device A 100 is the sync device.

Device A 100 and device B 200 respectively include TMDS transmissioncircuits 101 and 201. When device A 100 is the source device, the TMDStransmission circuit 101 functions as a transmitter, and the TMDStransmission circuit 201 functions as a receiver. On the other hand,when device B 200 is the source device the TMDS transmission circuit 201functions as the transmitter, and the TMDS transmission circuit 101functions as the receiver. The TMDS transmission circuit 101 is anexample of a transmission circuit or a first transmission circuit statedin the claims. Also, the TMDS transmission circuit 201 is an example ofa transmission circuit or a second transmission circuit stated in theclaims.

A serial transmission method based on TMDS is used in transmissionbetween the TMDS transmission circuits 101 and 201. Like in the HDMIstandard, video signals and sound signals are transmitted using threeTMDS channels 310 to 330. In other words, in a valid image period whichis a period from a vertical synchronizing signal to a subsequentvertical synchronizing signal excluding a horizontal retrace period anda vertical retrace period, a differential signal corresponding to pixeldata of an image of one uncompressed screen is transmitted to the syncdevice in one direction through the TMDS channels 310 to 330. Also, inthe horizontal retrace period or the vertical retrace period, adifferential signal corresponding to sound data, control data, otherauxiliary data, or the like is transmitted to the sync device in onedirection through the TMDS channels 310 to 330.

Like in the HDMI standard, a clock signal is transmitted through a TMDSclock channel 340. It is assumed that pixel data of 10 bits istransmitted through each of the TMDS channels 310 to 330 during oneclock transmitted through the TMDS clock channel 340.

Like in the HDMI standard, a display data channel (DDC) 350 is provided.The DDC 350 is originally used for a source device to read E-EDIDinformation stored in an EDID ROM 182 or 282 of the sync device. Whenthe sync device is a display device, the enhanced extended displayidentification data (E-EDID) information represents information relatedto settings and performance, such as a type, a resolution, colorcharacteristics, timing, and the like of the display device. The E-EDIDinformation is retained in the EDID ROM 182 or 282 of the sync device.Also, as registers of address space which can be accessed through theDDC 350, device A 100 and device B 200 respectively include DDCregisters 183 and 283.

Further, like in the HDMI standard, a consumer electronics control (CEC)line 361, a utility line 362, a hot plug detect (HPD) line 363, and thelike are provided. The CEC line 361 is a line for performingbidirectional communication of device control signals. While the DDC 350connects devices on a one-to-one basis, the CEC line 361 is shared amongall devices connected to an interface for communication.

The HPD line 363 is a line for detecting a connection (a hot plug) withanother device through the cable 300. As circuits which detect aconnection of the HPD line 363, device A 100 and device B 200respectively include plug connection detecting circuits 120 and 220. Ina HDMI 1.4 standard, an audio return channel (ARC) which transmits audiosignals as a synchronization signal from a sync device to a sourcedevice using the utility line 362 and the HPD line 363 is defined. Ascircuits which realize the ARC, device A 100 and device B 200respectively include in-phase signal communication circuits 160 and 260.

FIG. 2 is a diagram showing an example of a pin arrangement of aconnector in accordance with an embodiment of the present technology.This example of a pin arrangement is in accordance with correspondencerelationships between pin numbers 301 and signal names 302 which are Atype of the HDMI standard.

Each of the TMDS channels 310 to 330 and the TMDS clock channel 340includes three pins, that is, a positive electrode pin, a shield pin,and a negative electrode pin. Pins number one to number three, pinsnumber four to number six, pins number seven to number nine, and pinsnumber 10 to number 12 correspond to the TMDS channel 330, the TMDSchannel 320, the TMDS channel 310, and the TMDS clock channel 340,respectively.

Pins number 13, number 14, and number 19 correspond to the CEC line 361,the utility line 362, and the HPD line 363, respectively. Also, the DDC350 includes three pins, that is, a serial clock (SCL) pin, a serialdata (SDA) pin, and an earthing (ground) pin, which correspond to pinsnumber 15 to number 17 respectively. A ground connection (the pin number17) of the DDC 350 is also used as a ground connection of the CEC line361. A pin number 18 corresponds to a power supply line (+5V).

[Control for Reversal of Transmission Direction]

FIG. 3 is a diagram showing an example of a functional configurationrequired to control reversal of a transmission direction in accordancewith an embodiment of the present invention.

As described above, device A 100 includes the TMDS transmission circuit101. Device A 100 includes a transmission direction reversal controlunit 170 in order to control a transmission direction of the TMDStransmission circuit 101. Also, the aforementioned DDC register 183 isused in order to exchange information required to control thetransmission direction reversal control unit 170 with device B 200. Asdescribed above, device B 200 also includes the TMDS transmissioncircuit 201. Device B 200 includes a transmission direction reversalcontrol unit 270 in order to control a transmission direction of theTMDS transmission circuit 201. Also, the aforementioned DDC register 283is used in order to exchange information required to control thetransmission direction reversal control unit 270 with device A 100.

In the HDMI standard, a transmission direction of TMDS channel #0 310,TMDS channel #1 320, TMDS channel #2 330, and the TMDS clock channel 340(these four channels will be generally referred to as TMDS channelsbelow) is fixed in one direction. In contrast, this embodiment isconfigured to be able to appropriately reverse the transmissiondirection. In other words, when one of device A 100 and device B 200serves as the source device, the other one serves as the sync device. Atransmission direction of the TMDS channels is set to one direction fromthe source device to the sync device. Therefore, when device A 100 anddevice B 200 switch their roles, the transmission direction of the TMDSchannels are reversed between device A 100 and device B 200.

The DDC registers 183 and 283 are registers which can be accessedthrough the DDC 350. In this embodiment, TMDS transmission directionreversal requesting registers are defined in the DDC registers 183 and283 and accessed by the transmission direction reversal control unit 170of device A 100 and the transmission direction reversal control unit 270of device B 200 so that reversal of the transmission direction iscontrolled. Details of a control procedure according to thesetransmission direction reversal control units 170 and 270 will bedescribed below. The transmission direction reversal control unit 170 isan example of a transmission direction reversal control unit or a firsttransmission direction reversal control unit stated in the claims. Also,the transmission direction reversal control unit 270 is an example of atransmission direction reversal control unit or a second transmissiondirection reversal control unit stated in the claims.

FIG. 4 is a diagram showing examples of configurations of the TMDStransmission circuits 101 and 201 in accordance with an embodiment ofthe present invention. Here, description will be focused on TMDS channel#0 310 as a transmission path. TMDS channel #0 310 is made up of apositive electrode signal line 311, a negative electrode signal line312, and a shield signal line 313. This is the same for the other TMDSchannels.

The TMDS transmission circuit 101 includes amplifiers 111 and 112, adirect current bias circuit 113, and a shield terminal 114. Theamplifier 111 is an amplifier which amplifies an input signal. An outputof the amplifier 111 is a differential signal, and a positive electrodesignal and a negative electrode signal are respectively supplied to thepositive electrode signal line 311 and the negative electrode signalline 312. The amplifier 112 is an amplifier which amplifies a signalfrom the TMDS channels. The amplifier 112 operates due to a differentialinput, and the positive electrode signal line 311 and the negativeelectrode signal line 312 are input to an input of the positiveelectrode and an input of the negative electrode, respectively. Thedirect current bias circuit 113 is a circuit which places a bias voltageon the TMDS channels which are differential signal lines. In the HDMIstandard, the direct current bias circuit 113 is provided on a receiverside which receives a differential signal. The shield terminal 114 is aterminal for grounding and is connected to the shield signal line 313.

The TMDS transmission circuit 201 includes amplifiers 211 and 212, adirect current bias circuit 213, and a shield terminal 214. Aconfiguration thereof is identical to the configuration of the TMDStransmission circuit 101. Both of the direct current bias circuits 113and 213 are provided in this example, but it is also possible to provideonly one.

When a default configuration of the TMDS transmission circuit 101 is thesource device, the amplifier 112 may be alternating current(AC)-connected to a differential input through condensers 115. In thisway, it is possible to reduce a load on a transmission path during adefault operation. Likewise, when a default configuration of the TMDStransmission circuit 201 is the sync device, the amplifier 212 may beAC-connected to a differential output through condensers 215. In thisway, it is possible to reduce a load on a transmission path during thedefault operation.

FIG. 5 is a diagram showing an example of a configuration of device A100 in accordance with an embodiment of the present technology. FIG. 6is a diagram showing an example of a configuration of device B 200 inaccordance with an embodiment of the present technology. Here, a statein which device A 100 and device B 200 are connected through an SCL line351 and an SDA line 352 is shown. Also, ground lines are omitted.

Device A 100 includes a processing unit 181, the DDC register 183, aselector 184, and drivers 185 to 187. Likewise, device B 200 includes aprocessing unit 281, the DDC register 283, a selector 284, and drivers285 to 287.

When data is transmitted from device A 100 to device B 200, device A 100is a master, and “1” is output as a transmission direction signal DIRfrom the processing unit 181. With this, the driver 185 operates, andthe output of the driver 186 becomes high impedance. For this reason, aclock signal CLK from the processing unit 181 is output to the SCL line351. Also, the selector 184 selects a data signal DATA from theprocessing unit 181. For this reason, the data signal DATA is output tothe SDA line 352 through the driver 187.

In this case, device B 200 is a slave, and “0” is output as atransmission direction signal DIR from the processing unit 281. Withthis, the driver 286 operates, and the output of the driver 285 becomeshigh impedance. For this reason, a value of the SCL line 351 is input tothe DDC register 283 as a clock signal CK. Also, the selector 284connects a value of the SDA line 352 to the DDC register 283 as a datasignal D. For this reason, the DDC register 283 retains or outputs thedata signal D according to the clock signal CK.

On the other hand, when data is transmitted from device B 200 to deviceA 100, device B 200 is the master, and “1” is output as the transmissiondirection signal DIR from the processing unit 281. Also, in this case,device A 100 is the slave, and “0” is output as the transmissiondirection signal DIR from the processing unit 181. In other words,device A 100 and device B 200 perform operations opposite to the casedescribed above.

In this way, the DDC 350 is bidirectional, and information required tocontrol reversal of the transmission direction of the TMDS channels canbe exchanged between device A 100 and device B 200.

[TMDS Transmission Direction Reversal Requesting Register]

FIGS. 7a, 7b and 7c are a diagram showing an example of a format of aTMDS transmission direction reversal requesting register in accordancewith an embodiment of the present invention. This TMDS transmissiondirection reversal requesting register is defined in each of the DDCregisters 183 and 283. Whether or not the TMDS transmission directionreversal requesting register is valid and has a function of reversingthe transmission direction of the TMDS channels is defined in acapability declarative data structure of EDID and the like. Therefore,by referring to the EDID ROMs 182 and 282, it is possible to determinewhether or not the TMDS transmission direction reversal requestingregister has the function.

As shown in 7 a of the drawing, the TMDS transmission direction reversalrequesting register is assumed to be defined at an offset “0x0X” of theDDC registers 183 and 283. Also, attributes of access can be both ofread and write.

As shown in 7 b of the drawing, the TMDS transmission direction reversalrequesting register has a TMDS transmission direction reversalrequesting flag RevReq. When the transmission direction is changed froma forward direction to a backward direction, a TMDS transmissiondirection reversal requesting flag of a TMDS transmission directionreversal requesting register implemented in a sync device is used. Onthe other hand, when the transmission direction is changed from thebackward direction to a forward direction, a TMDS transmission directionreversal requesting flag of a TMDS transmission direction reversalrequesting register implemented in a source device is used.

When the TMDS transmission direction reversal requesting flag is set bythe sync device, the TMDS transmission direction reversal requestingflag is read by the source device and then cleared by the sync device.Also, activation is finished at a point in time at which the TMDStransmission direction reversal requesting flag is set by the sourcedevice.

As shown in 7 c of the drawing, the TMDS transmission direction reversalrequesting flag RevReq provided to the sync device and a TMDStransmission direction reversal requesting flag RevReqReturn provided tothe source device may be separately assigned.

[Operation]

FIG. 8 is a flowchart showing an example of a processing procedure whena source device dominantly controls reversal of a TMDS transmissiondirection in accordance with an embodiment of the present invention.Here, when device A 100 operates as a source device and device B 200operates as a sync device, it is assumed that device A 100 dominantlyreverses a transmission direction as the source device.

First, device A 100 determines whether or not device B 200 supports aTMDS transmission direction reversal control function by referring tothe EDID ROM 282 of device B 200 through the DDC 350 (step S911). As aresult, when device B 200 does not support the TMDS transmissiondirection reversal control function, reversal of the transmissiondirection is considered to be impossible and the process ends (stepS912: No). When device B 200 supports the TMDS transmission directionreversal control function (step S912: Yes), the following process isperformed.

By setting a TMDS transmission direction reversal requesting flag of aTMDS transmission direction reversal requesting resister in the DDCregister 283 of device B 200, device A 100 requests activation ofreversal (step S913). After this request, device A 100 opens the DDC 350and the TMDS channels and stops driving each signal line (step S921).Likewise, device B 200, which detects the reversal request from device A100, also opens the DDC 350 and the TMDS channels and stops driving eachsignal line (step S921). In this way, each configuration of device A 100and device B 200 changes, and device B 200 and device A 100 arerespectively configured as the source device and the sync device.

After device A 100 is configured as the sync device, device A 100accesses the HPD line 363 (step S922). In this way, device B 200 isnotified that it is possible to access the EDID ROM 182 of device A 100.

Device A 100 waits until device B 200 starts accessing the EDID ROM 182(step S923). When device B 200 accesses the EDID ROM 182, it is possibleto detect that the configuration of device B 200 has been changed to thesource device. When a predetermined time elapses until the EDID ROM 182is accessed (step S924: Yes), it is determined that the configuration ofdevice B 200 has not been changed to the source device. In this case,device A 100 cancels the request for activation of reversal and returnsto the configuration of the source device (step S928). After that,device A 100 waits for device B 200 to assert the HPD line 363 and endsthis process.

When device B 200 accesses the EDID ROM 182 within the predeterminedtime (step S924: No), it is checked in succession whether or not theTMDS transmission direction reversal requesting flag has been set in theDDC register 183 (step S925). At this time, if it is not possible tocheck that the TMDS transmission direction reversal requesting flag hasbeen set in the DDC register 183 even after a certain time (step S926:No), like the above description, device A 100 cancels the request foractivation of reversal and returns to the configuration of the sourcedevice (step S928).

When the TMDS transmission direction reversal requesting flag has beenset in the DDC register 183 (step S926: Yes), device B 200 becomes thesource device and starts transmission in which device A 100 isconsidered as the sync device (step S927).

FIG. 9 is a flowchart showing an example of a processing procedure whena sync device dominantly controls reversal of a TMDS transmissiondirection in accordance with an embodiment of the present invention.Here, when device A 100 operates as a source device and device B 200operates as a sync device, it is assumed that device B 200 dominantlyreverses a transmission direction as the sync device. This process isstarted, for example, when a user instructs device B 200 to play contentusing a remote controller of device B 200 or device A 100 is suppliedwith power and newly instructed to play sound in device A 100.

Device B 200 is a slave in DDC communication as the sync device, andthus first sets a value in a TMDS transmission direction reversalrequesting flag of a TMDS transmission direction reversal requestingregister implemented therein by itself (step S931). After that, device B200 requests device A 100 to read the TMDS transmission directionreversal requesting register of device B 200 (step S932). This readrequest may use a means of communication other than a DDC communicationpath or may be implemented by exchanging in advance special signalswhich can be recognized between device A 100 and device B 200 on a DDCcommunication path but are not prescribed in I2C communication.

After that, device B 200 waits until the TMDS transmission directionreversal requesting register is read by device A 100 (step S933). Atthis time, if reading is not performed even after a certain time (stepS934: Yes and step S935: No), this activation process is considered tohave failed and ends. In this case, for example, a message indicatingthat it is not possible to use the transmission direction reversalfunction may be displayed on a display for the user. Also, device B 200may make a read request again without ending the process.

When reading is performed by device A 100, device B 200 determines thatdevice A 100 has accepted the activation request from device B 200 andclears the TMDS transmission direction reversal requesting flag set bydevice B 200 (step S936).

Device A provides a response to the request from device B 200 by settingan activation request flag in the TMDS transmission direction reversalrequesting register of device B 200 in consideration of a capabilitythereof. This register may be identical to the aforementioned registeror may uniquely have a different address.

Device B 200 waits until device A 100 performs writing in the registeras a response to the activation request (step S937). At this time, ifwriting is not performed even after a certain time (step S938: Yes andstep S939: No), it is determined that device A 100 does not support theTMDS transmission direction reversal function, and the process ends. Inthis case, some messages may be displayed on the display for the user.

When writing is performed by device A 100 (step S939: Yes), it isdetermined that activation with device A 100 has been finished, and theprocess of step S941 and the following steps is executed. Since theprocess of step S941 and the following steps is identical to the processof step S921 and the following steps in the process described in FIG. 8,the description will be omitted here.

As described above, according to an embodiment of the presenttechnology, it is possible to control reversal of a transmissiondirection of a TMDS channel by requesting reversal of a TMDStransmission direction and responding to the request through the DDC350.

2. Application Example

In the embodiment described above, control for reversal of atransmission direction based on the HDMI standard has been described,but the present technology can also be applied to other configurations.As application examples of the present technology, examples ofconfigurations of an audiovisual system, a disc player, a televisionreceiver, and a digital camera will be described here.

FIG. 10a, 10b, 10c are a diagram showing an example of a configurationof an audiovisual system in accordance with an embodiment of the presenttechnology. As shown in 10 a of the drawing, this audiovisual systemincludes a disc player 10, an audio amplifier 20, and a televisionreceiver 30.

As shown in 10 b of the drawing, the disc player 10 plays contentrecorded on a disk medium and generates a video signal and an audiosignal. Then, the audio signal is played by the audio amplifier 20, andsound is output from a speaker (not shown). Also, the video signal isplayed by the television receiver 30 and displayed on a display of thetelevision receiver 30. In this case, the disc player 10 is a sourcedevice, and the audio amplifier 20 and the television receiver 30operate as sync devices.

On the other hand, when a broadcast signal received in the televisionreceiver 30 is played, as shown in 10 c of the drawing, there is a casein which a video signal is requested to be played as it is in thetelevision receiver 30 and an audio signal is requested to be played inthe audio amplifier 20. In this case, in relation to the audio signal,the television receiver 30 is the source device, and the audio amplifier20 operates as the sync device. For this reason, in an interface of therelated HDMI standard, it is not possible to respond to such a requestwithout physically breaking a connection because of a differenttransmission direction. In this respect, according to an embodiment ofthe present technology, it is possible to reverse the transmissiondirection without physically breaking the connection, and thus the audioamplifier 20 can play audio signals supplied from the televisionreceiver 30.

FIG. 11 is a diagram showing an example of a configuration of a discplayer 500 in accordance with an embodiment of the present technology.This disc player 500 includes a processor 511, a flash ROM 512, an SDRAM513, and a display control unit 514. Also, the disc player 500 includesa SATA interface 521, a BD drive 522, an Ethernet (registered trademark)interface 523, and a network terminal 524. Also, the disc player 500includes a high-speed bus interface 525, an HDMI transmitting unit 526,an HDMI terminal 527, and a remote control receiving unit 529. Also, thedisc player 500 includes an MPEG decoder 533, a video output terminal538, and a sound output terminal 539. Also, the disc player 500 includesa graphic generating circuit 542, a panel driving circuit 543, and adisplay panel 544. The processor 511, the flash ROM 512, the SDRAM 513,the display control unit 514, the SATA interface 521, the Ethernetinterface 523, and the MPEG decoder 533 are mutually connected via a bus510.

The processor 511 generally controls the disc player 500. The flash ROM512 is a memory which stores a program and the like required for theprocessor 511 to operate. The SDRAM 513 is a memory which stores dataand the like required for the processor 511 to operate.

The SATA interface 521 is an interface for connecting to a serial ATA.The BD drive 522 is a disk drive for driving a Blu-ray® disc. TheEthernet interface 523 is an interface for connecting to Ethernet. Thenetwork terminal 524 is a terminal for connecting to Ethernet.

The high-speed bus interface 525 is the utility line 362 and the HPDline 363 of the HDMI standard. The HDMI transmitting unit 526 is atransmission circuit for an interface of the HDMI standard. The HDMIterminal 527 is a terminal consistent with the HDMI standard.

The remote control receiving unit 529 receives a signal from a remotecontroller (not shown) manipulated by a user.

The MPEG decoder 533 is a decoder which decodes a signal in accordancewith the MPEG standard. The video output terminal 538 is a terminalwhich outputs video signals. The sound output terminal 539 is a terminalwhich outputs sound signals.

The display control unit 514 performs control required for display onthe display panel 544. The graphic generating circuit 542 generatesgraphic images to be displayed on the display panel 544. The paneldriving circuit 543 is a circuit for driving the display panel 544. Thedisplay panel 544 is a panel which displays video. Here, the displaypanel 544 is assumed to be built into the disc player 500, but the discplayer 500 may not have the built-in display panel 544.

The high-speed bus interface 525 is inserted between the Ethernetinterface 523 and the HDMI terminal 527. The high-speed bus interface525 transmits transmission data, which is supplied from the processor511, from the HDMI terminal 527 to a device on the other side through anHDMI cable. Also, the high-speed bus interface 525 supplies receptiondata, which is received from the device on the other side through theHDMI cable and the HDMI terminal 527, to the processor 511.

During content recording, content data to be recorded is acquiredthrough a digital tuner (not shown), the Ethernet interface 523, or thehigh-speed bus interface 525. The content data is input to the SATAinterface 521 and recorded in a BD by the BD drive 522. The content datamay be recorded in a hard disk drive (HDD) (not shown), which isconnected to the SATA interface 521.

During content playback, content data (an MPEG stream) which is playedfrom a BD by the BD drive 522 is supplied to the MPEG decoder 533through the SATA interface 521. In the MPEG decoder 533, a decodingprocess is performed on the played content data, and baseband image andsound data is obtained. The image data is output to the video outputterminal 538 through the graphic generating circuit 542. Also, the sounddata is output to the sound output terminal 539.

Also, during the content playback, the image data obtained in the MPEGdecoder 533 is supplied to the panel driving circuit 543 through thegraphic generating circuit 542 in accordance with a user manipulation,and playback images are displayed on the display panel. Also, audio dataobtained in the MPEG decoder 533 is supplied to a speaker (not shown) inaccordance with a user manipulation, and sound corresponding to theplayback images is output.

Also, during the content playback, if the image and sound data istransmitted through HDMI TMDS channels, the image and sound data issupplied to the HDMI transmitting unit 526 and packed. Then, the imageand sound data is output from the HDMI transmitting unit 526 to the HDMIterminal 527.

Also, during the content playback, if the content data played by the BDdrive 522 is sent to a network, the content data is output to thenetwork terminal 524 through the Ethernet interface 523. Likewise,during the content playback, if the content data played by the BD drive522 is sent to a bidirectional communication path of the HDMI cable, thecontent data is output to the HDMI terminal 527 through the high-speedbus interface 525. Before the image data is output, the image data maybe encrypted using a copyright protection technology, for example, HDCP,DTCP, DTCP+, or the like, and then transmitted.

FIG. 12 is a diagram showing an example of a configuration of atelevision receiver 600 in accordance with an embodiment of the presenttechnology. This television receiver 600 includes a processor 611, aflash ROM 612, an SDRAM 613, and a display control unit 614. Also, thetelevision receiver 600 includes an Ethernet interface 623 and a networkterminal 624. Also, the television receiver 600 includes a high-speedbus interface 625, an HDMI receiving unit 626, an HDMI terminal 627, anda remote control receiving unit 629. Also, the television receiver 600includes an antenna terminal 631, a digital tuner 632, and an MPEGdecoder 633. Also, the television receiver 600 includes a video signalprocessing circuit 641, a graphic generating circuit 642, a paneldriving circuit 643, and a display panel 644. Also, the televisionreceiver 600 includes a sound signal processing circuit 651, a soundamplifier circuit 653, and a speaker 654. The processor 611, the flashROM 612, the SDRAM 613, the display control unit 614, the Ethernetinterface 623, the high-speed bus interface 625, and the MPEG decoder633 are mutually connected via a bus 610.

The processor 611 generally controls the television receiver 600. Theflash ROM 612 is a memory which stores a program and the like requiredfor the processor 611 to operate. The SDRAM 613 is a memory which storesdata and the like required for the processor 611 to operate. The displaycontrol unit 614 performs control required for display on the displaypanel 644.

The Ethernet interface 623 an interface for connecting to Ethernet. Thenetwork terminal 624 is a terminal for connecting to Ethernet. Thehigh-speed bus interface 625 is the utility line 362 and the HPD line363 of the HDMI standard. The HDMI receiving unit 626 is a transmissioncircuit for an interface of the HDMI standard. The HDMI terminal 627 isa terminal consistent with the HDMI standard.

The remote control receiving unit 629 receives a signal from a remotecontroller (not shown) manipulated by a user.

The antenna terminal 631 is a terminal to which a television broadcastsignal received by a receiving antenna (not shown) is input. The digitaltuner 632 processes the television broadcast signal input from theantenna terminal 631 to extract a partial TS from a predeterminedtransport stream corresponding to a channel selected by the user.

The MPEG decoder 633 is a decoder which decodes a signal in accordancewith the MPEG standard. The MPEG decoder 633 performs a decoding processon video PES packets, which include TS packets of video data obtained bythe digital tuner 632, to obtain image data. Also, the MPEG decoder 633performs a decoding process on sound PES packets, which include TSpackets of sound data obtained by the digital tuner 632, to obtain sounddata.

The video signal processing circuit 641 and the graphic generatingcircuit 642 perform a signal process as necessary on the image dataobtained by the MPEG decoder 633 or image data received at the HDMIreceiving unit 626. This signal process is assumed to be, for example, ascaling process (a resolution conversion process), a graphics datasuperimposing process, gamma correction of a WCG image, and the like.The panel driving circuit 643 is a circuit for driving the display panel644 on the basis of video (image) data output from the graphicgenerating circuit 642. The display panel 644 is a panel which displaysvideo. The display panel 644 may be configured with, for example, aliquid crystal display (LCD), a plasma display panel (PDP), an organicelectro-luminescence (EL) panel, and the like.

In this embodiment, an example including the display control unit 614 inaddition to the processor 611 is shown, but the processor 611 maydirectly control a display on the display panel 644. Also, the processor611 and the display control unit 614 may be configured as one chip ormay be a plurality of cores.

The sound signal processing circuit 651 performs a necessary process,such as D/A conversion and the like, on the sound data obtained by theMPEG decoder 633. The sound amplifier circuit 653 amplifies a soundsignal output from the sound signal processing circuit 651 and suppliesthe amplified sound signal to the speaker 654. The speaker 654 may bemonaural or stereo. The speaker 654 may be one, or two or more innumber. The speaker 654 may be earphones or headphones. The speaker 654may correspond to 2.1 channels, 5.1 channels, or the like. The speaker654 may be wirelessly connected to the television receiver 600. Thespeaker 654 may be another device.

The television broadcast signal input to the antenna terminal 631 issupplied to the digital tuner 632. The digital tuner 632 processes thetelevision broadcast signal and outputs the predetermined transportstream corresponding to the channel selected by the user. Then, thepartial TS (the TS packets of video data and the TS packets of sounddata) is extracted from the transport stream and supplied to the MPEGdecoder 633.

In the MPEG decoder 633, a decoding process is performed on the videoPES packets, which include TS packets of video data, to obtain the videodata. At the video signal processing circuit 641 and the graphicgenerating circuit 642, the video data is subjected to a scaling process(a resolution conversion process), a graphics data superimposingprocess, and the like as necessary and then supplied to the paneldriving circuit 643. For this reason, images corresponding to thechannel selected by the user are displayed on the display panel 644.

Also, in the MPEG decoder 633, a decoding process is performed on thesound PES packets, which include TS packets of sound data, to obtain thesound data. The sound data is subjected to a necessary process, such asD/A conversion and the like, in the sound signal processing circuit 651,is also amplified in the sound amplifier circuit 653, and then issupplied to the speaker 654. For this reason, sound corresponding to thechannel selected by the user is output from the speaker 654.

Content data (image data and sound data) which is supplied from thenetwork terminal 624 to the Ethernet interface 623 or supplied from theHDMI terminal 627 through the high-speed bus interface 625 is suppliedto the MPEG decoder 633. After that, the same operation as theabove-described operation during reception of a television broadcastsignal is performed so that images are displayed on the display panel644 and sound is output from the speaker 654.

At the HDMI receiving unit 626, image data and sound data transmittedfrom the disc player 500, which is connected to the HDMI terminal 627through an HDMI cable, is acquired. The image data is supplied to thevideo signal processing circuit 641. Also, the sound data is directlysupplied to the sound signal processing circuit 651. After that, thesame operation as the above-described operation during the reception ofthe television broadcast signal is performed so that images aredisplayed on the display panel 644 and sound is output from the speaker654.

FIG. 13 is a diagram showing an example of a configuration of a digitalcamera 700 in accordance with an embodiment of the present technology.This digital camera 700 includes a system control processor 711, a flashROM 712, an SDRAM 713, and a display control unit 714. Also, the digitalcamera 700 includes a high-speed bus interface 725, an HDMI transmittingunit 726, an HDMI terminal 727, and a user manipulation unit 728. Also,the digital camera 700 includes a graphic generating circuit 742, apanel driving circuit 743, and a display panel 744. Also, the digitalcamera 700 includes an imager 751, an imager driver 752, and a cameracontrol unit 753. Also, the digital camera 700 includes an imagingsignal processing circuit 754, a microphone 755, a sound signalprocessing circuit 756, a still image signal processing circuit 757, amoving image signal processing circuit 758, a recording and playbackunit 759, and a memory card 760.

The system control processor 711 generally controls the digital camera700. The flash ROM 712 is a memory which stores a program and the likerequired for the system control processor 711 to operate. The SDRAM 713is a memory which stores data and the like required for the systemcontrol processor 711 to operate. The display control unit 714 performscontrol required for display on the display panel 744.

The high-speed bus interface 725 is the utility line 362 and the HPDline 363 of the HDMI standard. The HDMI transmitting unit 726 is atransmission circuit for an interface of the HDMI standard. The HDMIterminal 727 is a terminal consistent with the HDMI standard.

The user manipulation unit 728 receives manipulation of a user. The usermanipulation unit 728 may be, for example, a switch, a wheel, a touchpanel unit which inputs an instruction by an approach/touch, a mouse, akeyboard, a gesture input unit which detects an input of an instructionwith a camera, a sound input unit which inputs an instruction withvoice, a remote control, and the like. The system control processor 711determines a manipulation state of the user manipulation unit 728 andcontrols operation of the digital camera 700. Through the usermanipulation unit 728, the user can perform an input manipulation andthe like for various kinds of additional information in addition to animaging (recording) manipulation and a playback manipulation.

The graphic generating circuit 742 generates graphic images to bedisplayed on the display panel 744. The panel driving circuit 742 is acircuit for driving the display panel 744. The display panel 744 is apanel which displays video.

The imager 751 is a photoelectric conversion device which converts areceived optical signal into an electric signal. The imager driver 752is a driver which drives the imager 751. The camera control unit 753controls the imager driver 752 and the imaging signal processing circuit754. The imaging signal processing circuit 754 performs a signal processon the electric signal supplied from the imager 751 as an imagingsignal.

The microphone 755 converts ambient sound to generate a sound signal.The sound signal processing circuit 756 performs a signal process on thesound signal generated by the microphone 755.

The still image signal processing circuit 757 performs a signal processon a still image signal supplied from the imaging signal processingcircuit 754. The moving image signal processing circuit 758 performs asignal process on a moving image signal supplied from the imaging signalprocessing circuit 754 and the sound signal processing circuit 756.

The recording and playback unit 759 accesses the memory card 760, theflash ROM 712, or the like and reads still image data or moving imagedata. The memory card 760 is a recording medium in which the still imagedata or moving image data is recorded.

An imaging signal obtained by the imager 751 is supplied to the imagingsignal processing circuit 754 and processed, and image data (taken imagedata) corresponding to a subject is obtained from the imaging signalprocessing circuit 754. When a still image is taken, a compressionencoding process and the like is performed on the image data output fromthe imaging signal processing circuit 754 in the still image signalprocessing circuit 757, and still image data is generated. The stillimage data is recorded in the memory card 760 and the like by therecording and playback unit 759.

Also, when moving images are taken, a compression encoding process andthe like in accordance with a recording media format is performed on theimage data output from the imaging signal processing circuit 754together with sound data output from the sound signal processing circuit756 in the moving image signal processing circuit 758, and moving imagedata to which sound data is added is generated. The moving image data isrecorded in the memory card 760 and the like by the recording andplayback unit 759.

When a still image is played, still image data is read from the memorycard 760 or the like and subjected to a process, such as decoding andthe like, in the still image signal processing circuit 757, and playbackimage data is obtained. The playback image data is supplied to the paneldriving circuit 743 through the system control processor 711 and themoving image signal processing circuit 758, and the still image isdisplayed on the display panel 744.

When moving images are played, moving image data is read from the memorycard 760 or the like by the recording and playback unit 759 andsubjected to a process, such as decoding and the like, in the movingimage signal processing circuit 758, and playback image data isobtained. Then, the playback image data is supplied to the panel drivingcircuit 743, and the moving images are displayed on the display panel744.

When image or sound data associated with still image data or movingimage data recorded in the memory card 760 and the like is transmittedto an external device (a sync device), the still image data or movingimage data is read from the memory card 760. Then, the read still imagedata or moving image data is supplied to the still image signalprocessing circuit 757 or the moving image signal processing circuit 758and subjected to a process, such as decoding and the like, and basebandimage and sound data is obtained. Then, the baseband image and sounddata is supplied to the HDMI transmitting unit 726 and sent to an HDMIcable connected to the HDMI terminal 727.

Also, when image or sound data associated with still image data ormoving image data directly output from the imaging signal processingcircuit 754 or the sound signal processing circuit 756 is transmitted tothe external device (the sync device), the image or sound data issupplied to the HDMI transmitting unit 726. Then, the image or sounddata is sent to the HDMI cable connected to the HDMI terminal 727.

The above-described embodiments are examples for embodying the presenttechnology, and matters in the embodiments each have a correspondingrelationship with disclosure-specific matters in the claims. Likewise,the matters in the embodiments and the disclosure-specific matters inthe claims denoted by the same names have a corresponding relationshipwith each other. However, the present technology is not limited to theembodiments, and various modifications of the embodiments may beembodied in the scope of the present technology without departing fromthe spirit of the present technology.

The processing sequences that are described in the embodiments describedabove may be handled as a method having a series of sequences or may behandled as a program for causing a computer to execute the series ofsequences or recording medium storing the program. As the recordingmedium, a CD (Compact Disc), an MD (MiniDisc), and a DVD (DigitalVersatile Disk), a memory card, and a Blu-ray® disc can be used.

In addition, the effects described in the present specification are notlimiting but are merely examples, and there may be other effects.

Additionally, the present technology may also be configured as below.

-   (1)

An interface circuit including:

a transmission circuit configured to perform transmission to anotherdevice through a transmission path; and

a transmission direction reversal control unit configured to open thetransmission path after making a request for reversal of a transmissiondirection in the transmission path to the other device, and to reversethe transmission direction at the transmission circuit in response topermission for reversal from the other device.

-   (2)

The interface circuit according to (1),

wherein, while the transmission circuit transmits data to the otherdevice, the transmission direction reversal control unit makes therequest for reversal to the other device.

-   (3)

The interface circuit according to (1) or (2),

wherein, when the request for reversal is received from the other devicewhile the transmission circuit transmits data to the other device, thetransmission direction reversal control unit makes the request forreversal to the other device.

-   (4)

The interface circuit according to any one of (1) to (3),

wherein the transmission circuit includes an amplifieralternating-current connected to a differential input from thetransmission path.

-   (5)

An interface circuit including:

a transmission circuit configured to perform transmission to anotherdevice through a transmission path; and

a transmission direction reversal control unit configured to open thetransmission path in response to a request for reversal of atransmission direction in the transmission path from the other deviceand, when the other device becomes accessible again, give the otherdevice permission for reversal and reverse the transmission direction atthe transmission circuit.

-   (6)

The interface circuit according to (5),

wherein, while the transmission circuit receives data from the otherdevice, the transmission direction reversal control unit receives therequest for reversal from the other device.

-   (7)

The interface circuit according to (5) or (6),

wherein, while the transmission circuit receives data from the otherdevice, the transmission direction reversal control unit makes therequest for reversal to the other device.

-   (8)

The interface circuit according to any one of (5) to (7),

wherein the transmission circuit includes an amplifieralternating-current connected to a differential output to thetransmission path.

-   (9)

A transmission system including:

a first device; and

a second device, the first device and the second device performingtransmission to each other through a transmission path, wherein

the first device includes

-   -   a first transmission circuit configured to perform transmission        to the second device through the transmission path, and    -   a first transmission direction reversal control unit configured        to open the transmission path after making a request for        reversal of a transmission direction in the transmission path to        the second device, and to reverse the transmission direction at        the first transmission circuit in response to permission for        reversal from the second device, and

the second device includes

-   -   a second transmission circuit configured to perform transmission        to the first device through the transmission path, and    -   a second transmission direction reversal control unit configured        to open the transmission path in response to the request for        reversal from the first device, and, when the first device        becomes accessible again, to give the first device the        permission for reversal and to reverse the transmission        direction at the second transmission circuit.

-   (10)

A transmission direction control method including:

a procedure of making a request for reversal of a transmission directionin a transmission path to another device that is a target for whichtransmission is performed from a transmission circuit through thetransmission path;

a procedure of opening the transmission path; and

a procedure of reversing the transmission direction at the transmissioncircuit in response to permission for reversal from the other device.

-   (11)

A transmission direction control method including:

a procedure of receiving a request for reversal of a transmissiondirection in a transmission path from another device that is a targetfor which transmission is performed from a transmission circuit throughthe transmission path;

a procedure of opening the transmission path; and

when the other device becomes accessible again, a procedure of givingthe other device permission for reversal and reversing the transmissiondirection at the transmission circuit.

-   (12)

A program for causing a computer to perform:

a procedure of making a request for reversal of a transmission directionin a transmission path to another device that is a target for whichtransmission is performed from a transmission circuit through thetransmission path;

a procedure of opening the transmission path; and

a procedure of reversing the transmission direction at the transmissioncircuit in response to permission for reversal from the other device.

-   (13)

A program for causing a computer to perform:

a procedure of receiving a request for reversal of a transmissiondirection in a transmission path from another device that is a targetfor which transmission is performed from a transmission circuit throughthe transmission path;

a procedure of opening the transmission path; and

when the other device becomes accessible again, a procedure of givingthe other device permission for reversal and reversing the transmissiondirection at the transmission circuit.

REFERENCE SIGNS LIST

-   10 disc player-   20 audio amplifier-   30 television receiver-   101, 201 transition minimized differential signaling (TMDS)    transmission circuit-   111, 112, 211, 212 amplifier-   113, 213 direct current bias circuit-   114, 214 shield terminal-   120, 220 plug connection detecting circuit-   160, 260 in-phase signal communication circuit-   170, 270 transmission direction reversal control unit-   181, 281 processing unit-   182, 282 EDID ROM-   183, 283 display data channel (DDC) register-   184, 284 selector-   185 to 187, 285 to 287 driver-   300 cable-   310 to 330 TMDS channel-   311 positive electrode signal line-   312 negative electrode signal line-   313 shield signal line-   340 TMDS clock channel-   350 display data channel (DDC)-   351 SCL line-   352 SDA line-   361 consumer electronics control (CEC) line-   362 utility line-   363 HPD line-   500 disk player-   600 television receiver-   700 digital camera

The invention claimed is:
 1. An interface circuit, comprising: a firstregister configured to store a first request flag; a transmissioncircuit configured to transmit data to an external device via atransmission path; and a transmission direction reversal control unitconfigured to: set a second request flag in a second register of theexternal device to request reversal of a direction of transmission ofthe data in the transmission path; open the transmission path based onthe set second request flag; detect a change in configuration of theexternal device, wherein the change in the configuration is detectedbased on a start of access of the first register by the external devicewithin a specific time period; and reverse the direction of thetransmission of the data at the transmission circuit based on the firstrequest flag in the first register that is accessed by the externaldevice, and the detected change in the configuration of the externaldevice, wherein the transmission circuit includes an amplifieralternating-current connected to a differential input from thetransmission path.
 2. The interface circuit according to claim 1,wherein the second request flag is set based on the transmission of thedata via the transmission circuit to the external device.
 3. Theinterface circuit according to claim 1, wherein the second request flagis set based on a reception of a request for the reversal of thedirection by the external device during the transmission of the data tothe external device.
 4. An interface circuit, comprising: a transmissioncircuit configured to: transmit first data to an external device througha transmission path; and receive second data from the external device;and a transmission direction reversal control unit configured to: detecta request flag in a register of the interface circuit for reversal of adirection of transmission of the first data in the transmission path,wherein the request flag is set in the interface circuit by the externaldevice during reception of the second data by the interface circuit fromthe external device, the direction of the transmission of the first datain the transmission path is reversed from a first direction to a seconddirection, the first direction is from the external device to theinterface circuit for the reception of the second data by the interfacecircuit from the external device, and the second direction is from theinterface circuit to the external device for the transmission of thefirst data by the interface circuit to the external device; open thetransmission path based on the detected request flag; and reverse thedirection of the transmission of the first data at the transmissioncircuit based on a start of access of the register by the externaldevice within a specific time period, wherein the transmission circuitincludes an amplifier alternating-current connected to a differentialinput from the transmission path.
 5. The interface circuit according toclaim 4, wherein the transmission direction reversal control unit isfurther configured to receive a request for the reversal of thedirection from the external device during the reception of the seconddata.
 6. The interface circuit according to claim 4, wherein thetransmission direction reversal control unit is further configured totransmit a request for the reversal of the direction to the externaldevice during the reception of the second data.
 7. A transmissionsystem, comprising: a first device; and a second device, wherein thefirst device includes: a first register configured to store a firstrequest flag; a first transmission circuit configured to transmit firstdata to the second device through a transmission path; and a firsttransmission direction reversal control unit configured to: set a secondrequest flag in a second register of the second device to requestreversal of a direction of transmission of the first data in thetransmission path; open the transmission path based on the set secondrequest flag; detect a change in configuration of the second device,wherein the change in the configuration is detected based on a start ofaccess of the first register by the second device within a specific timeperiod; and reverse the direction of the transmission of the first dataat the first transmission circuit based on  the first request flag inthe first register that is accessed by the second device, and  thedetected change in the configuration of the second device, and thesecond device includes: a second transmission circuit configured to:receive the first data from the first device through the transmissionpath; and transmit second data to the first device through thetransmission path; and a second transmission direction reversal controlunit configured to: detect the second request flag for the reversal ofthe direction of the transmission of the first data in the transmissionpath; open the transmission path based on the detected second requestflag; accept the detected second request flag based on an accessibilityof the first device; and reverse the direction of the transmission ofthe first data at the second transmission circuit based on the detectedsecond request flag, wherein the transmission circuit includes anamplifier alternating-current connected to a differential input from thetransmission path.
 8. A transmission direction control method,comprising: storing a first request flag in a first register;transmitting data to an external device via a transmission path; settinga second request flag in a second register of the external device torequest reversal of a direction of transmission of the data in thetransmission path; opening the transmission path based on the set secondrequest flag; detecting a change in configuration of the externaldevice, wherein the change in the configuration is detected based on astart of access of the first register by the external device within aspecific time period; and reversing the direction of the transmission ofthe data at a transmission circuit based on the first request flag inthe first register that is accessed by the external device, and thedetected change in the configuration of the external device, wherein thetransmission circuit includes an amplifier alternating-current connectedto a differential input from the transmission path.
 9. A transmissiondirection control method, comprising: in an interface circuit:transmitting first data to an external device through a transmissionpath; receiving second data from the external device; detecting arequest flag in a register of the interface circuit for reversal of adirection of transmission of data in the transmission path, wherein therequest flag is set in the interface circuit by the external deviceduring reception of the second data by the interface circuit from theexternal device, the direction of the transmission of the first data inthe transmission path is reversed from a first direction to a seconddirection, the first direction is from the external device to theinterface circuit for the reception of the second data by the interfacecircuit from the external device, and the second direction is from theinterface circuit to the external device for the transmission of thefirst data by the interface circuit to the external device; opening thetransmission path based on the detected request flag; and reversing thedirection of the transmission of the first data at a transmissioncircuit based on a start of access of the register by the externaldevice within a specific time period, wherein the transmission circuitincludes an amplifier alternating-current connected to a differentialinput from the transmission path.
 10. A non-transitory computer-readablemedium having stored thereon computer-executable instructions, whichwhen executed by a computer, cause the computer to execute operations,the operations comprising: storing a first request flag in a firstregister; transmitting data to an external device via a transmissionpath; setting a second request flag in a second register of the externaldevice to request reversal of a direction of transmission of the data inthe transmission path; opening the transmission path based on the setsecond request flag; detecting a change in configuration of the externaldevice, wherein the change in the configuration is detected based on astart of access of the first register by the external device within aspecific time period; and reversing the direction of the transmission ofthe data at a transmission circuit based on the first request flag inthe first register that is accessed by the external device, and thedetected change in the configuration of the external device, wherein thetransmission circuit includes an amplifier alternating-current connectedto a differential input from the transmission path.
 11. A non-transitorycomputer-readable medium having stored thereon computer-executableinstructions, which when executed by a computer of an interface circuit,cause the computer to execute operations, the operations comprising:transmitting first data to an external device through a transmissionpath; receiving second data from the external device; detecting arequest flag in a register of the interface circuit for reversal of adirection of transmission of data in the transmission path, wherein therequest flag is set in the interface circuit by the external deviceduring reception of the second data by the interface circuit from theexternal device, the direction of the transmission of the first data inthe transmission path is reversed from a first direction to a seconddirection, the first direction is from the external device to theinterface circuit for the reception of the second data by the interfacecircuit from the external device, and the second direction is from theinterface circuit to the external device for the transmission of thefirst data by the interface circuit to the external device; opening thetransmission path based on the detected request flag; and reversing thedirection of the transmission of the first data at a transmissioncircuit based on a start of access of the register by the externaldevice within a specific time period, wherein the transmission circuitincludes an amplifier alternating-current connected to a differentialinput from the transmission path.